Digital X-ray detectors, are used, inter alia, in medical diagnostics. The size of these detectors is typically between 20×20 cm2 and 43×43 cm2. The current state of the art is represented by detectors based upon amorphous silicon (using indirect conversion) and amorphous selenium (with direct conversion). The principles for direct conversion (I) and indirect conversion (II) are set out in FIG. 1. In direct conversion I, an X-ray quantum 1 excites a particle 2 and electron/hole pairs 2a, 2b are formed which then migrate to the electrodes 4 (anode and cathode, for example, pixel electrodes) and are detected there. In indirect conversion II, the X-ray quantum 1 excites the particle 2 which, in turn, emits radiation 2′ with a lower energy (e.g. visible light, UV, or IR radiation) which is then detected by means of a photodetector 3 (e.g. photodiode).
Indirect X-ray conversion comprises the combination of a scintillator layer (e.g. Gd2O2S or CsI with different dopants such as terbium, thallium, europium, etc.; layer thicknesses are typically 0.1-1 mm) and a photodetector (e.g. a photodiode). The emission wavelength of the scintillator light from X-ray conversion coincides with the spectral sensitivity of the photodetector.
In the case of direct X-ray conversion, again the X-ray radiation is directly converted into electron/hole pairs and these are electronically read out (e.g. amorphous Se). Direct X-ray conversion in selenium is typically undertaken with up to 1 mm thick layers which are reverse biased in the kV range. Whereas indirectly converting detectors have become established in particular due to being easy and inexpensive to manufacture, direct converters have a significantly better resolving power.
An alternative to the aforementioned X-ray detectors on the basis of inorganic semiconductors is hybrid organic detectors which have conventionally been manufactured through application from the liquid phase. This enables, in particular, easy processing on large areas of up to 43×43 cm2 or more. The manufacturing of detectors conventionally comprises the introduction of the inorganic absorber materials, for example, quantum dots or typical scintillator materials, into an organic matrix. Organic semiconductors can easily be applied from the liquid phase onto large surfaces and through the direct mixing-in of the inorganic scintillator granules, the optical cross-talk can be significantly minimized.
Organic semiconductors, in contrast to inorganic semiconductors, have a lower conductivity. This restricted conductivity is problematic if, as for example in X-ray absorption, very thick layers are needed to achieve a sufficient level of sensitivity. Firstly, the efficiency of the photodiode is thereby reduced, since the charge carrier extraction is impeded, secondly the speed of the photodiode is lowered, which limits a use for medical devices, for example, to the field of mammography where only soft X-ray radiation with a low penetration depth is used.
Organic semiconductors are mainly applied from the liquid phase or vapor deposited in a vacuum. All the methods known to date for mixing in inorganic absorber materials use processing from the liquid phase.
U.S. Pat. No. 6,483,099 B1 describes the possibility of an X-ray detection with a scintillator layer on an OPD (organic photodiode). Further embodiments are X-ray detection by mixing (“admixture”) of scintillators into an OPD, scintillator as substrate or as part of the electrode. No information is given as to how a scintillator can be introduced homogeneously into a thick OPD layer or how an e.g. 100 μm-thick hybrid diode can be manufactured.
DE 101 37 012 A1 discloses an embodiment of a light-sensitive polymer absorber layer with embedded scintillator granules. The conductivity of the polymer layer is increased by the absorption of light from the scintillator. The mean spacing of the scintillator granules in the layer corresponds to the mean free path length of the photons from the scintillator in the polymer.
DE 10 2008 029 782 A1 describes an X-ray detector based on quantum dots mixed into the organic semiconductor matrix. In this concept, the quantum dots are dispersed in the organic semiconductor solution. Herein, oleic acid or similar is used, which can influence the electrical properties of the organic semiconductor.
DE 10 2010 043 749 A1 relates to an X-ray detector based on the concept described above, wherein scintillators are either dispersed directly in the organic semiconductor solution or are sprayed on in a co-spraying process simultaneously with the organic semiconductor material.